A headset type microphone is worn on the head of a user for a long period of time. For the reason, the headset microphone is different from a hand-held type microphone that it is often suffered from liquid such as sweat and spittle of the user. In addition, the headset microphone is often submerged in water of for example, swimming pool by sports instructors. As a result, such the liquid sometimes adversely affects the headset microphone.
Besides the headset microphone, for example, "Karaoke" (or orchestra) microphone is used in entertainment facilities where beverages such as beer and juice are served. For the reason, such the microphone is suffered from the beverage dashed thereon, and erroneous dropping thereof into the beverage.
Accordingly, as shown in FIG. 5, in a conventional accommodation of a mike unit 2 into an unit accommodating chamber 1a of a mike casing 1, a sound receiving opening portion 1b of the mike casing 1 is covered with a waterproof membrane 3, and an O-ring 5a is interposed between a support arm 4 of a headset and the mike casing 1 so that the unit accommodating chamber 1a is kept airtightly.
In this instance, the waterproof membrane 3 comprises a thinner plastic film made of materials such as polyethylene, and polyethylene terephtalate, which does not seriously affect vibration energy of sound wave passing therethrough so that excellent sensitivity, frequency characteristic and directivity are produced.
Furthermore, to provide an excellent acoustic characteristics, the waterproof membrane 3 is put on a support ring 3a in such a way not to be pulled by large tension applied thereto, and covers over a sound receiving opening portion 1b through the support ring 5a. In addition, an O-ring 5b is also provided between the support ring 3a and the mike casing 1.
However, the waterproof membrane 3 and the O-rings 5a, 5b seal the interior of the unit accommodating chamber 1a almost completely. So, the waterproof membrane 3 undergoes pressure differential between the interior of the unit accommodating chamber and the outside of the mike casing due to the changes in external atmospheric pressure and internal temperature and the like. consequently, the waterproof membrane 3 has a resonant point under tension. Theerefore, the acoustic characteristics become lessened. The possibility arises that the waterproof membrane 3 will be broken.
To prevent this, the internal volume of the unit accommodating chamber is increased. When the sound receiving portion cannot be formed in larger size, a bypass passage then brings the sound receiving portion into communication with a large volume portion which is separately formed within the mike casing, as proposed, for example, in Japanese Utility Model Publication No. 7-28793 Publication.
However, any of objectives of methods mentioned above is to maintain airtightness of the interior of the unit accommodating chamber. Therefore, basically, the problem of the pressure differential between the interior of the unit accommodating chamber and the outside of the mike casing is not solved. In order to solve the problem, it is contemplated to provide permeability for the waterproof membrane itself. However, an acoustically desirable material is even yet not found such the waterproof membrane.
Such the problem will be accomplished by the present invention. Accordingly, an object of the invention is to provide a waterproof type microphone which equalizes the pressures between the interior of an unit receiving chamber and the outside of a mike casing without degradation of the waterproof performance and acoustic characteristics.